The antenna of a satellite receiving station is conventionally constituted by a parabolic reflector. This reflector is most often of circular or ovoid shape. In all cases, the reception principle remains the same: the electromagnetic waves are focused onto the reception focus. The signal is received by a "source" and then amplified by the microwave frequency head.
There are several types of parabolic antennas. The three main types of antennas are as follows:
The antenna of the type having symmetry of revolution, or the "Prime focus" type whose microwave frequency head is supported by a tripod fixed to the outer edges of the parabola, and directly placed in the proximity of the focus of the reflector: the presence of the head in the active part of the parabola results in a masking effect and in diffraction phenomena. A waveguide (feeder) is sometimes used to convey the signal from the source to the microwave frequency head (placed at the rear in this case). PA1 The "Cassegrain" type antenna whose microwave frequency head is installed at the rear of the main reflector and receives the waves reflected on a hyperbolic sub-reflector which reconcentrates the signals received by the main reflector towards the low noise amplifier (called LNA "Low Noise Amplifier"); this sub-reflector is the generator of a masking effect. PA1 The antenna of the offset illumination type which is a parabolic antenna having an off centered focus: the low noise amplifier and the source are offset in such a way as to reduce the masking effect. PA1 the receiving gain of a flat antenna is on average 25% lower than that of a parabolic antenna, the bandwidth is limited and in order to receive the 2 circular polarisations, 2 antennas are necessary; PA1 the production cost of such an antenna is high, principally for two reasons: PA1 1. expensive materials are necessary in order to minimise losses; PA1 2. the matrix represented by the receiving area of the antenna requires an individual connection of each of the microelements. PA1 right circular polarization (or "right hand circular polarisation") for example TDF1, BSB, BS, OLYMPUS; PA1 left circular polarization (or "left hand circular polarization") for example TVSAT, OLYMPUS. PA1 horizontal linear polarisation: for example Intelsat V, ECS 1 F1 PA1 vertical linear polarization: for example Intelsat V, ECS 1 F1, Telecom 1; the channels of the satellites of the "Eutelsat" and "Intelsat" organizations allocated to television transmissions are divided into two sub-groups of programmes, each of which corresponds to a different polarization.
The choice of the type of antenna principally depends on the size of the microwave frequency head used: a voluminous microwave frequency head, if it is placed at the center of the parabola, reduces its gain. Furthermore, the materials used for constructing parabolic reflectors are principally of the plastic or metallic (aluminium) type. Finally, the diameter of the parabola is a function of the merit factor, G/T, linking the gain G of the parabola and the overall noise temperature (T) desired on the station. This diameter has been able to decrease considerably in recent years, with a constant G/T, because of technological improvements in the amplifiers which result in a reduction of their noise temperature. The diameter of the parabola defines its beam width and, in addition to their discretion, the major advantage of parabolas of small diameter is the ease of aiming because of the corresponding increase in beam width. However, at the same time, the beam width determines the sensitivity of the system to interference coming from satellites adjacent to the satellite aimed at, which limits the possible reduction in diameter.
Furthermore, most manufacturers are currently developing array antennas, called flat antennas, intended either for receiving television transmissions, or for communications, mobile or fixed, for data transmission for professional use: the entire area of the antenna receives the radio signals transmitted by the satellite; an array of receiving microelements is placed in parallel and the gain is a function of the area of the antenna.
The efficiency of such flat antennas reduces considerably when the antenna area increases because of the loss generated in the summing systems.
But, the use of a flat antenna is capable of simplifying the procedures and therefore of limiting the costs of installations: a flat antenna can be installed almost vertically on a wall, or stuck to a roof. It merges with the decor (greater aesthetic qualities given it by its design): small thickness, reduced dimensions (it is configured in the form of squares of side 35 to 70 cm), lightness, discretion;
In order to be capable of receiving programmes from several satellites a flat antenna should be motorised. Array antennas with electronic aiming are being developed. They will allow the reception of transmissions from several adjacent satellites without movement. But no production for the general public is known at present as each microelement must be controlled in phase which considerably affects the gain and the noise temperature of the antenna.
Furthermore, in the current state of the art, these flat antennas have three big disadvantages:
With regard to the microwave frequency heads, these are composed of two elements: a low noise amplifier (LNA) and a low noise converter (LNC for Low Noise Converter), which can be connected as independent modulators one after the other, or integrated in a single block (LNB: Low Noise Block down converter).
Furthermore, the frequency bands allocated to television transmissions by geostationary satellite have been optimized to release a sufficient number of channels for all potential users (countries and international organizations). Consequently, satellites of the DBS type use two types of electromagnetic radiation polarized in opposite directions. Two transmissions can thus coexist on a same channel: their opposite polarizations enable them to be separated on reception.
The polarisations used by the two types of television satellites are as follows.
For direct television broadcast satellites:
For telecommunications satellites:
When a receiving station is intended to receive several types of polarizations, depolarizers are provided in order to allow the user to choose the desired polarization at will. The choice of these systems depends on the nature of the polarizations received.
In the case of horizontal and vertical polarisations, it is necessary to have recourse to a motorized polarization changing system, most often mounted on the waveguide of the parabola.
In the case of right and left circular polarizations (direct television broadcast satellites), the simultaneous reception of polarized signals requires a double output waveguide, equipped with an orthomode transducer allowing the mounting on a same parabola of two microwave frequency heads each dedicated to a different polarization.
Motorized systems intended to ensure the switching of the microwave frequency heads at the center of the parabola are now developed, but the reliability of these equipments is insufficient.
Very wide band heads, which are multi-frequency by band switching or agile frequency synthesis, should allow the reception of all of the frequencies allocated to television transmissions. But such multi-band heads will not quickly be available at reasonable prices.
The output of the microwave frequency head is connected to a demodulator which converts and demodulates the signal received in the satellite intermediate band (BIS) 950 to 1750 MHz. The demodulator allows the selection of the satellite channels to be carried out. Only demodulators having a very wide input band, which cover the entire frequency range from 950 to 1750 MHz, are capable of receiving the transmissions from all of the satellites which will cover Europe in the coming years. At present, these demodulators are only used to the maximum of their capabilities in motorized stations intended for the reception of several satellites.